1. Field of the Invention
The present invention relates to a method for correcting a mask pattern, a computer program product, a method for producing a photomask, and a method for manufacturing a semiconductor device. In particular, it relates to a correction of optical proximity effects.
2. Description of the Related Art
Accompanying the miniaturization of semiconductor devices in recent years, various lithography technologies have been developed making it now possible to obtain fine patterns. For instance, using modified illumination or a phase-shifting photomask, it has become possible to resolve patterns having a pattern dimension value normalized with an exposure light wavelength λ and a numerical aperture NA of a projection optical system of well below 0.5.
Under such photolithography conditions, a phenomenon of being unable to transfer a mask pattern of a photomask onto a wafer as designed, namely the Optical Proximity Effect (OPE), has become apparent. Optical Proximity Correction (OPC), which is a technique for matching a pattern shape that is transferred onto a wafer, to the original design, has become important.
Through the introduction of the OPC technique, it has become possible to control variations in critical dimensions (CD) on the wafer. As a result, a fine pattern may be faithfully fabricated as designed on the wafer. Accordingly, the mask pattern on the photomask has become remarkably different from corresponding designed pattern on the wafer.
Until now, “a rule based correction method” and “a model based correction method” have been proposed as the OPC technique.
With the rule based correction method, mask pattern correction amounts corresponding to mask pattern placement are made into a rule table beforehand. Correction proceeds based on the mask pattern placement information while referencing the rule table. The rule table is normally produced through test results. With the rule based correction method, although the correction process is simple, it is difficult to generate all of the actual variations in circuit patterns into a rule table.
With the most simplified rule based correction method, the amount of correction is in accordance with a distance between neighboring mask patterns. With a general optical system, even though distances between mask patterns are the same, if line widths of the mask patterns differ, it has ben shown theoretically that light intensity distributions on the wafer are different during transfer. Accordingly, if the amount of correction is coordinated as a single-value function of the distance between the neighboring mask patterns, sufficient accuracy for the correction may not be possible.
Methods that complexify design rules in order to improve correction accuracy are also being studied. However, the number of rules increases with rule complexification and therefore the procedure for correction is also made more complicated. In addition, problems develop such as in deciding how the rules themselves should be obtained.
Meanwhile, the model based correction method predicts the shape that will be transferred onto the wafer based on mask pattern information and wafer process conditions; and then adds corrections to the mask pattern to obtain desired values. With the model based correction method, to begin with, evaluation points are allocated and edges are partitioned for input mask pattern data. Light intensity calculations on adjacent evaluation points are then performed and the amount of deviation from the transferred pattern edge location on the wafer is calculated. Then, the amount of mask pattern correction for each partitioned edge is found in accordance with the amount of deviation. The edges are then shifted, transforming the mask pattern. Deviation evaluation and mask pattern correction are then repeated on a post-transformation mask pattern. If the amount of deviation has been brought down below a certain level, correction is ended.
Here, it is important to decide how much shift to apply to the mask pattern in response to the amount of deviation. Although the mask pattern is normally only shifted by an amount proportional to the amount of deviation, it is not easy to set a proportionality coefficient. The amount of deviation of the transferred pattern on the wafer changes depending on the mask pattern shape. As a result, the deviation evaluation and the mask pattern correction is usually repeated several times.
Highly accurate correction becomes possible if the calculation of light intensity is performed for deviation evaluation using a precision model. However, using the precision model requires a long time for the deviation evaluation. Consequently, it takes a long time to perform the correction. In general, there are many cases where a rough model having low accuracy is used to implement a high speed calculation, sacrificing a certain degree of calculation accuracy.
As described above, while highly accurate correction may be carried out with the model based correction method using the precision model with a high accuracy, the time required for correction increases. Accordingly, it is difficult to obtain a desired level of correction accuracy within a practical length of correction time.